Paralysis in spinal cord injury is largely due to loss of functional corticospinal motor neurons (CSMN), the cortical projection neurons that control voluntary movement, and do not spontaneously regenerate. Transplantation of embryonic stem (ES) cells that can be differentiated into immature cortical projection neurons in vitro is considered a potential therapeutic strategy. However, these cells arrest at intermediate stages of development and co-express transcription factor markers of multiple cortical projection neuron lineages, the molecular controls over which are still unknown. I propose a novel approach to the problem of CSMN development based on my preliminary observations that the microRNA (miRNA) miR-409-3p is enriched in CSMN-containing layers of the developing telencephalon and controls CSMN cell fate in embryonic cortical culture. I posit that miR-409-3p favors the development of CSMN by repressing transcription factors specific to other projection neuron subtypes, notably LMO4 and CITED2. I will test this interpretation by using three convergent experimental approaches: expression analysis, loss of function effects, and gain of function modifications. I am ideally suited to make significant contributions to this field because of my basic science background in genetics and nervous system development combined with my clinical training in spine surgery. My ultimate goal is to become independent neurosurgeon-scientist studying stem cell transplantation, recruitment of endogenous progenitor cells, and re-activation of developmental programs in injured CSMN as potential strategies for therapeutic regeneration following spinal cord injury. The training plan outlined in this application will permt me to develop additional expertise in stem cell culture, differentiation, and transplantation, and in small RNA analysis, which is needed in order to achieve these goals. Successful completion of the proposed studies will advance our understanding of cortical projection neuron subtype specification, inform therapeutic stem cell transplantation strategies, and enable the development of novel molecular therapies for spinal cord injury treatment. It will provide proof of principle for the use of miRNAs to manipulate cortical progenitor cell fates, thereby enabling the development of novel miRNA based therapies for spinal cord injury, while also catalyzing my transition to independence.